1. Field of the Invention
The present invention relates to a method of and an apparatus for detecting the presence or absence of foreign matter such as, for example, metallic pieces in products which have been packaged or have not yet been packaged (hereinafter referred to as merely xe2x80x9cproductxe2x80x9d).
2. Description of the Prior Art
The prior art apparatus for detecting the presence or absence of foreign matter includes a transport conveyors for successively transporting products to be inspected from one station towards another station past an inspecting station, and two detecting coils disposed at the inspecting station in a side-by-side fashion along the direction of transport of the product for producing an electromotive force as each product passes through the inspecting station. The presence or absence of foreign matter such as metallic pieces in the products is detected by differentially amplifying the electromotive force produced by the detecting coils during the passage of the product past the inspecting station, and detecting the phase of the electromotive force to provide a detected phase signal (hereinafter referred to as a xe2x80x9creaction signalxe2x80x9d). This prior art foreign matter detecting apparatus is so designed and so configured as to utilizes the principle in which the value of a maximum dip point (the point of the lowest voltage) and the phase of this maximum dip point differ depending on whether the product contains no metallic piece or whether the product contains metallic pieces.
The method of setting the point of phase determination and adjusting the detecting sensitivity employed in the prior art foreign matter detecting apparatus will be discussed in detail with reference to FIG. 7 showing a phase output characteristic representative of the level of the reaction signal with respect to the phase.
At the outset, while a sample product free from the foreign matter is transported by the transport conveyor past the inspecting station, the phase of the maximum dip point d of the reaction signal B1 produced as a result of the passage of the sample product past the inspecting station is detected. The phase so detected is preset as a phase determination point p, that is, predetermined phase point at which the level of the reaction signal is detected for the purpose of determining the presence or absence of the foreign matter. Then, while the same sample product is again transported by the transport conveyor past the inspecting station, the value of the reaction signal B2, produced as a result of the passage of the same sample product past the inspecting station, at the phase determination point preset in the manner described above is detected, wherefore the sensitivity of a detector can be automatically adjusted to a value effective to detect the reaction signal so that the value of the reaction signal B2 at the phase determination point p may effectively attain a value lower than a threshold value SH that is utilized to determine the presence or absence of the foreign matter.
However, the prior art method of adjusting the detecting sensitivity is such that the detecting sensitivity is merely adjusted for the purpose of avoiding the possibility that the product free from the foreign matter is erroneously determined as containing the foreign matter, and the minimum size of the foreign matter that can be detected has not been disclosed. For this reason, the minimum size of the foreign matter that can be detected varies depending on types of products to be inspected and, hence, the criterion warranted by a foreign matter detecting machine may vary.
In view of the foregoing, the present invention has been devised to substantially eliminate the above discussed problem inherent in the prior art foreign matter detecting apparatus and is intended to provide an improved foreign matter detecting apparatus in which the detecting sensitivity can be easily adjusted to a value effective to detect the foreign matter of a desired size.
In order to accomplish the foregoing object, the present invention provides a method of detecting the presence or absence of foreign matter in a product by using a reaction signal generating circuit for generating, while the product is passed through an alternating magnetic field, a reaction signal associated with the product based on the magnetic field influenced by the product. This method includes the steps of obtaining a first test reaction signal by passing a product, added with foreign matter of a minimum size to be detected, through the alternating magnetic field; and adjusting a sensitivity of the reaction signal generating circuit so that the value of the first test reaction signal at a predetermined phase point attains a desired level relative to a predetermined threshold value.
According to the method described above, the sensitivity of the reaction signal generating circuit can be so adjusted that the reaction signal associated with the product added with the foreign matter of the minimum size to be detected will not lower below the threshold value, but the reaction signal associated with the product added with no foreign matter will lower below the threshold value and, therefore, the presence of the foreign matter of the minimum size to be detected can be detected assuredly.
The present invention also provides an apparatus for detecting the presence or absence of foreign matter in a product, which includes a reaction signal generating circuit for outputting a reaction signal associated with the product by detecting a change in pattern of distribution of magnetic fluxes when the product is passed through an alternating magnetic field; a determining circuit for comparing a value of the reaction signal at a predetermined phase point with a threshold value to thereby determine the presence or absence of the foreign matter in the product; and a sensitivity adjusting means for adjusting a sensitivity of the reaction signal generating circuit so that the value of a first test reaction signal at the predetermined phase point, which is obtained when a product added with foreign matter of a minimum size to be detected is passed through the alternating magnetic field, attains a desired level relative to the threshold value.
According to the structure described above, since the sensitivity of the reaction signal generating circuit can be so adjusted that the reaction signal associated with the product added with the foreign matter of the minimum size to be detected will not lower below the threshold value, but the reaction signal associated with the product added with no foreign matter will lower below the threshold value, the presence of the foreign matter of the minimum size to be detected can be detected assuredly.
In a preferred embodiment of the present invention, in order to automatically set the predetermined phase point in the determining circuit, the phase at which the value of the second test reaction signal obtained when the product containing no foreign matter is passed through the alternating magnetic field attains a minimum value is set at the predetermined phase point or, alternatively, the phase at which the difference in level between the first and second reaction signals is maximized is set at the predetermined phase point.
In another preferred embodiment of the present invention, an amplifier may be employed in the foreign matter detecting apparatus for amplifying an output from the reaction signal generating circuit and outputting the reaction signal, in which case the sensitivity adjusting means is operable to adjust an amplification factor of the amplifier.